The modern automotive industry is striving to reduce or entirely eliminate the noises produced by vehicles. Today, essentially mats of fibrous insulating materials or open-pore foams are used for sound absorption and are mounted around the sources of noise or in the immediate vicinity thereof. However, the use of such open-pore sound absorbers in the engine space, as described, for example, in DE-34 28 157, proves to be problematic because they become contaminated with oil, water, dust and other impurities and thus rapidly decline in their acoustic effect.
For example, DE-40 11 705, DE-42 41 518 or DE-43 05 281 has therefore also already proposed providing an oil- and water-resistant arrangement comprising a large number of Helmholtz resonators. These known arrangements consist of box-like hollow bodies which have a hole or a neck. The volume of the hollow bodies together with the dimension of the hole or neck determines the resonant frequency of the absorber. These known arrangements are designed essentially for a frequency range from 1 to 2 kHz and can be mounted on the engine bonnet, in the wheel casing or on the floor.
These arrangements furthermore occupy an undesirably large amount of space, i.e. they cannot be used where there is a shortage of space.
When absorbers of this type are used in practice, the walls of the box-like hollow body must be light-weight, i.e. must be constructed very thin. However, these thin-walled hollow bodies tend to become deformed as a result of the acoustic pressure fluctuations and thus to limit the quality factor of the resonator. Since the quality factor plays a substantial role in determining the efficiency of the absorbers, it is also always necessary to accept a reduction in the acoustic efficiency of these absorbers when the light-weight construction method is used. The acoustic efficiency of these absorbers is in principle limited because the number of orifices which pick up the sound is limited by the geometric size of the individual hollow bodies. Typically, these hollow bodies have a base area of 15.times.15 mm.sup.2 to 60.times.60 mm.sup.2 in conjunction with an overall height of 5 to 25 mm and a hole diameter of 4 to 11 mm. It is therefore clear that these Helmholtz resonators can couple to the interfering sound field only to a limited extent since, when they are used extensively, the orifice area which is proportional to the quality factor Q and picks up the sound can be at most only 2.5% to 4% of the total area exposed to soundwaves. In addition, when the Helmholtz absorbers described are installed on a vehicle floor, the orifices are directed upwards and the cavities can therefore easily fill with moisture and dirt, which in turn impairs the sound absorption.
DE-39 13 347 also discloses an insulating part which has a large number of cell-like cavities which are arranged close together and are open on one side. By means of this insulating part, the energy of the incident sound field is essentially dissipated by irregular reflections, absorption in the material and interference effects. These insulating parts, too, are suitable only to a limited extent for use in automotive construction, in particular because they are readily soiled and, owing to their lack of intrinsic stability, rapidly wear out.